1. Field of the Invention
The present invention relates to a stacked memory module in mirror image arrangement and method for the same, especially to a stacked memory module with two memory units on upper face and lower face of a rigid printed circuit board and two flexible printed circuit board on both sides of the rigid printed circuit, and method for the same.
2. Description of the Prior Art
The processing speed of computer device increases as the progress of technology. The performance of memory is important beside the processing speed of CPU. The conventional computer memory modules generally comprise a plurality of memory units (memory ICs) arranged on carrier (such as printed circuit board) in array fashion. The development trend for memory module is compact and lightweight.
Multi-layer stacked memory module is a design for compact size and high capacity. Taiwan patent gazette No. 091123613 and No. 091221281 disclose that a memory unit is stacked to another memory unit, where the corresponding pins of the memory unit are electrically connected and signal can be conveyed therebetween. The pins of the memory units are toward the same direction. The memory module is packaged by board-on-chip packages and this technology is suitable for high speed Dynamic Random Access Memory (DRAM) or other memories. This package is especially suitable for Dual Inline Memory Module (DIMM) and provides improvement for pin connection between two memory units.
However, the signal lines for the stacked memory have non-uniform length and the load impedance might be increased with the stacked structure. The signal integrity is influenced and the signal is not stable. Moreover, the pins of two memory units should have mirror symmetry when the stacked structure is applied to the DIMM memory. The cost is increased due to circuit design and package requirement.